Coilers having a rotating mandrel require control of the pressing roller in response to signals which are generated in conjunction with the respective position of the forward or leading strip edge of the rolled strip which rotates with the mandrel.
The control function is carried out by electro-hydraulic positioning piston-and-cylinder units which actuate the auxiliary strip pressure rolls with concurrent regulation of the operating force and position.
The aim of control devices for this purpose is to overcome problems which arise during coiling of the initial layers or turns of rolled strip on the mandrel. In particular those problems result because the leading edge of the strip can cause a radial step, or hump or similar projection. This projection affects the subsequently wound strip turns due to the high pressure which is exerted by the pressure rolls on the strip layers by causing shock or dynamic stresses between the coil and the pressure rolls. Such extensive damage may be caused to the first few turns of the coil that they are so severely marked as to require these portions of the strip to be treated as scrap material.
The control device which has been described in German Patent Publication DE-AS No. 2,158,721 operates in such a way that the pressure rolls are temporarily lifted during the strip winding or coiling. Lifting is done by a distance equal to the thickness of the strip, i.e. the height of the radial hump which is caused by the leading edge of the strip, when this hump passes the pressure roll.
The lifting motion is controlled by a pulse generator which is coupled to the coiler mandrel, and the pulse generator feeds a pulse counter which produces signals, by way of a control logic, for the control device. In response to the signals, the control device actuates the piston and cylinder units connected at the pressure roll when the radial rise passes this roll.
In a further known device of this type, the path of the leading edge of the strip accumulating on the coiler mandrel is monitored during formation of the first strip winding by way of contactless sensors, and the path is further monitored and recorded by means of electronic recorders. The positioning of the rolls is then controlled by means of the recorded information using interpreter/evaluating devices.
In the two prior art devices, the up and down motions of the control elements for the piston and cylinder units, which effect the lifting and contacting movements of the pressure rolls, are controlled by two independent electro-hydraulic control circuits or loops, namely, a position control loop, and a pressure control loop.
The lifting movement of the pressure roll is then effected and controlled by utilization of the corresponding position value in the position control loop. The contacting movement and the subsequent pressing of the roller against the strip is correspondingly effected by input and utilization of a setpoint pressure value in the pressure control loop. The pressure control loop is open when position control is effected.
The prior art devices, accordingly, have the drawback that they require constant and very rapid switching between position control and pressure control. Furthermore, this type of control causes rocking or jerking (hunting) of the system, and the dynamic behavior of the elements which take part in the operation is detrimentally affected. During position control a further problem arises in the case of the known devices in that either no signal may be generated for the respective control device, or only incorrect position signals may be generated. In such an event, the respective piston and cylinder unit will not change its position, and it will not carry out the required movement to avoid an impact against the oncoming rise, for example. Accordingly, additional overload safety equipment must be employed.